The Republican former Speaker of the House has joined the advisory board of Acreage Holdings, a company that cultivates, processes and dispenses cannabis in 11 U.S. states. Boehner's endorsement, after saying nine years ago he was "unalterably opposed" to legalization, could be considered a watershed event: Marijuana has gone mainstream.

"Over the last 10 or 15 years, the American people's attitudes have changed dramatically,"...